The present invention relates broadly to safety apparatus for electric power systems, and more specifically to such apparatus for use with underground mining apparatus, such as coal transport or shuttle cars, which are electrically powered via conductors from a remote power supply. Particularly, the invention functions to cut off direct current supplied to a mine car if a conductive portion of the car body or frame has become effectively shorted to a power conductor.
In underground mining apparatus such as transport cars which are electrically powered via a cable from a remote power supply, a potential hazard occurs if one of the power conductors in a cable or one of the load elements of the car should become electrically shorted to a conductive portion of the vehicle body or frame. Because the vehicle body or frame is not earth grounded, circuit breakers will not be actuated under these conditions, and the vehicle body will become electrically hot, with attendant hazard to workers in the mine.
In the operation of mine cars it is desirable to disconnect the power supply from the cable on the loss of continuity in the electrical cable which delivers power to the car. National Mine Service Co. of Pittsburgh, Pa. manufactures and distributes circuitry, known by the tradename Femco GM1000 Ground Sentinel II, for monitoring three-phase alternating current power cables for continuity and for disconnecting the power supply on the occurrence of a discontinuity. The Ground Sentinel II includes a 4 KHz alternating current generator, circuitry associated with the generator to sense the impedance of the generator's load at 4 KHz, and a switched output which causes a contactor to disconnect the power supply when the generator's load impedance raises above a predetermined level. In use with a three-phase alternating current power system, the generator and monitoring circuitry are connected to the power cable at the power station by a three phase filter and to the car via a pilot or ground wire. A second three-phase filter at the car couples the car body to the cable. If any cable conductor should open, the load impedance for the 4 KHz generator increases; when this condition is sensed the circuit drives the contactor, thereby disconnecting the three phase power supply. The Femco circuit does not monitor the cable for short circuits.